Composite structures of laminated material are increasingly being used in industry, and, particularly, in the aircraft industry. From time to time, the composite structure will be damaged and need repair in the field rather than replacement of an entire panel or subassembly in depot maintenance. Repair typically involves the removal of damaged material and covering the repair site with layers of woven material, such as graphite or carbon fiber, which have been impregnated with an organic matrix resin, such as epoxy. The new material and an adhesive, if necessary, are pressed into place and cured at an elevated temperature. Pressure conforms the new patch material to the original structure and beat properly sets the resin. When correctly done, such curing involves a controlled heating profile to a predetermined temperature which is held for a sufficient time to complete the resin's curing reaction followed by a controlled cooling profile.
The advantages of designing with composite materials include the ability to tailor the amount of material used to obtain efficient structural components. Many composite designs have been developed that have nonuniform cross-sections (i.e., ply dropoffs, planks, stiffening elements, etc.). The heat sinks from these non-uniform cross-sections require increased thermal control to maintain uniform cures. Curing of resins used in composite materials (including those in the repair patches) is an exothermic reaction that requires heat to start the reaction. Without adequate control of the heating or cooling, hot spots or cold spots develop in the repair. Conventional heat blankets and control techniques that seek to reduce cold spots tend to increase problems associated with hot spots or visa versa. The autoclave provides elevated positive laminating pressures and the large thermal mass needed for precise temperature control. Existing portable repair equipment has neither the elevated pressures nor the inherent temperature control capabilities of the autoclave. Consequently, repairs to complex structures are often inadequate because of poor temperature control and nonuniform temperatures in the repair zone, thereby reducing the structural capability of the repair.
Controlling the pressure applied and the temperature profile for a repair is important to the strength of repair. Inadequate temperature control can substantially impact repair strength. Heating too fast can shock and weaken the composite structure. Curing temperatures lower than desired result in poor bonding and temperatures higher than desired can result in burning both the repair patch and the material surrounding the repair. Fluctuating temperatures, especially during the cure, can produce a combination of these effects.
A repair site may be heated by placing electrical resistance heating blankets directly over the entire repair area. Such heating blankets tend to heat the site unevenly, however either because portions of the structure conduct heat away from the site while other portions retain heat or because in the heater provides variable uneven heat. In one study, we heated an aircraft skin by direct contact with a heating blanket and detected a 95.degree. F. (52.degree. C.) temperature variation under the blanket when the skin was heated to a nominal 360.degree. F. (183.degree. C.). In a second study, where we inserted a copper foil heat conductor between the heating blanket and the aircraft skin, we still found a 65.degree. F. (35.degree. C.) variation. Proper curing is impossible with such a large temperature variation.
The difficulties of achieving uniform heating are compounded by uneven thermal characteristics at a repair site. For example, a portion of a repair site may consist of aircraft skin only, while another portion may consist of aircraft skin having an underlying support member i.e., a spar. The support member tends to transfer heat away from the skin to which it is attached causing uneven temperatures across the repair site. Multi-zone heating blankets have been used to provide different amounts of heat to different portions of a repair site in an attempt to provide uniform temperatures to areas having varying thermal characteristics. Such multi-zone heating blankets, however, require complex control systems for the multiple heating zones and, importantly, must be specifically configured to provide heating characteristics corresponding to the thermal characteristics of different areas of the repair site. Providing complex controls and many different configurations of multi-zone heating blankets is a problem for repair centers. The repair surface must be mapped with respect to its thermal characteristics to know what regions to heat to what temperature and at what power to achieve overall temperature uniformity in the structure. This mapping is difficult and is a function of the environment where the repair is done.
Another system for repairing composite structures is disclosed in U.S. Pat. No. 4,808,253 to Mimbs. The Mimbs system uses a heating blanket on top of a thermally conductive fluid-filled envelope to distribute heat and pressure to the repair site. As disclosed, the envelope is primarily used to conform the repair assembly to contoured shapes. The Mimbs system does not result in a sufficiently accurate temperature control. Overlaying the envelope with a conventional heating blanket creates multiple heat transfer interfaces resulting in inefficient and inconsistent heat transfer to the fluid in the envelope. Mimbs lacks a thermometer to monitor the temperature of the repair site.
A need exists for composite repair apparatus which can achieve controlled heating and sufficiently sustain substantially uniform temperatures for appreciable periods of time to achieve proper bonding between the patch material and the structure being repaired.